Tissue invasion and metastasis, one of the six purported capabilities acquired by human cancers, accounts for more than 90% of cancer deaths and represents an understudied but promising area for future therapeutic developments. The long-term goal of our program is to understand how malignant tumor cells acquire the invasive and metastatic phenotype. In the next five years, we plan to focus on the microenvironment of tumor cell surface and test the hypothesis that cell surface proteolysis regulates the invasive and metastatic properties of malignant tumors. Current evidence suggest that proteinases contribute to tumor invasion and metastasis by not only degrading the extracellular matrix as a barrier, but also functioning to regulate pathways controlling cell growth, migration and apoptosis through releasing latent growth factors or cleaving various receptors and their ligands for both activation and inactivation. Yet, efforts targeting tumor proteinases especially the MMPs have not achieved any clinical success so far. Several outstanding reviews have recently been published to address this apparent gap between "scientific success and clinical failure" for the MMP field. We would like to argue that one neglected area is proteolvsis on tumor cell surface. Our evidence both in vitro and in vivo suggests that the same proteinase behaves differently when it is tethered on cell surface or secreted. We hypothesize that the membrane-bound MMPs are more efficient for proteolysis and harder to inhibit than soluble ones, thus, enabling tumor invasion and metastasis. To test this idea, we designed three specific aims: 1) Characterize the invasive and metastatic phenotype conferred by MT1-MMP expressed on tumor cell surface both in vitro and in vivo;2) Determine the contributions of the hemopexin- and catalytic- domains of MT1-MMP towards the invasive and metastatic phenotype;and 3) Characterize the microenvironment on tumor cell surface that enables MT1-MMP to mediate invasion and metastasis. Accomplishment of these aims may empower the design of a new generation of MMP inhibitors targeting the tumor cell surface and provide model systems to test the efficacies of these potential therapeutics.